what are some of the most necessary, realistic and economically feasible changes that we need to make (in your lifetime) to sustain our planet and the people on it.

There are many changes that can be done in order to achieve a greener, more sustainable lifestyle that doesn't damage the environment. Some of the most accesible ones, in terms of practicality and easy economic access, for everyone are the following:

1) Swift to a plant-based diet. This is the most instant, most efficient change we can do to diminish environment-damage. The meat and dairy industries are responsible for over 80% of the gases responsible for the green house effect and other environment-damaging practices, such as: deforestation; ocean poisoning, and ozone cape-thinning. You may do the following changes in your diet to avoid contributing to the explotation of land, contamination of the oceans and thinning of the cape:

Adopt at least one meal a week that does not contain animal-derived products. Doctors like Michael Greger advise to increase the consumption of raw greens (with regards to a typical American-diet) to recieve the right amounts of protein, iron and alcium, among others.
Incorporate more grains and wholefoods to your diet. This will provide you with plenty of nutrients and energy, and cleanse your body from potential cancerous cells.

2) Adopt zero-waste habits. Meaning, try not to consume foods or goods that come in packages made of plastic or other unrecyclable materials, or materials like, as plastic, take over 200 years to desintegrate.

Use cloth napkins and cloth shopping-bags.
Purchase reusable sandwich wraps or tuppers.
Purchase herbs to make tea using a tea globe infuser.
Use glass recipients. (they also take some time to desintegrate, but they are a better choice than plastic ones which do not last as long.)
Try to walk to your destination or use non-polluting means of transport like bycicles.

It is important not to buy these items if you do not need them right away: otherwise you are going to have to dispose of the ones you already have. Try to be consistent and implement at least one animal-product free day, or meal, and also take in more zero-waste habits.